Dense graphite structures

ABSTRACT

This disclosure describes a graphite composition and a fabrication procedure for producing coolant and/or support plates for acid fuel cells. The composition consists of a graphite powder sized principally in the 50 to 150 micron range with about seven percent of the graphite powder in the range below 50 microns. The process involves a controlled low-pressure heating step and an intermediate temperature full-pressure molding operation. Homogeneous graphite structures of high density with improved structural, thermal, and electrical properties can be provided in a variety of extremely intricate shapes by molding.

United States Patent 1191 Emanuelson et al.

( 1 Aug. 28, 1973 DENSE GRAPHITE STRUCTURES [73] Assignee: United Aircraft Corporation, East Hartford, Conn.

22 Filed: Mar. 25, 1971 211 Appl.No.: 127,979

6 Division of se'rfmffisifio, nuts", T669, Pat.

[52] US. Cl. 260/38 [51] int. Cl C08g 51/08 [58] Field of Search 260/38; 264/105; 136/122 [56] References Cited UNITED STATES PATENTS 2,556,830 6/1951 Thrune 260/38 X Ohlgren 260/38 X Henderson et al 260/38 X Primary Examiner-Lewis T. Jacobs AttorneyLaurence A. Savage [57] ABSTRACT This disclosure describes a graphite composition and a fabrication procedure for producing coolant and/or support plates for acid fuel cells. The composition consists of a graphite powder sized principally in the 50 to 150 micron range with about seven percent of the graphite powder in the range below 50 microns. The process involves a controlled low-pressure heating step and an intermediate temperature full-pressure molding operation. Homogeneous graphite structures of high density with improved structural, thermal, and electrical properties can be provided in a variety of extremely intricate shapes by molding.

4 Claims, 2 Drawing Figures FIG! MIX THERMOSETTING PHENOLIC RESIN BINDER WITH A LIQUID DISPERSANT SELECT GRAPHITE PARTICLES BLEND GRAPHITE PARTICLES WITH BINDER AND DISPERSANT DRY THE MIXTURE AND LOAD THE MOLD APPLY LESS THAN IOO PSI PRESSURE AND HEAT THE MOLD AND CONTENTS TO ABOUT 200 F APPLY ABOUT3000 PSI FULL PRESSURE WHEN TEMPERATURE REACHES ABOUT 0 200 F AND HEAT TO BETWEEN 300-40OF COOL THE MOLD AND CONTENTS WHILE FULL PRESSURE IS MAINTAINED INVENTORS ROGER C. EMANUELSON WARREN L. LUOMA ATTORNEY DENSE GRAPHITE STRUCTURES CROSS REFERENCE TO RELATED APPLICATIONS This application is a division of our copending appli- 5 cation, Ser. No. 789,780 filed on Jan. 8, 1969, now U. S. Pat. No. 3,634,569, both applications being assigned to the same assignee.

BACKGROUND OF THE INVENTION This invention relates to dense graphite structures and the method for fabricating such structures. In particular, this invention relates to molding dense graphite plates having complicated shapes for use in acid fuel cells.

The preparation of graphite structures using powder metallurgy techniques and any of a number of commercially available grades of carbon or graphite powders is known. Carbon or graphite bodies have been proposed and utilized in acid fuel cells but generally as porous bodies. It is known to provide carbon bodies from a fine powder of carbonaceous material with or without binders.

Graphite structures for use in acid electrolyte fuel cells have previously been machined from graphite blanks to the desired configuration. Experience has shown that this method is very expensive and the structural configurations are limited because of machining requirements. Further, problems are encountered in obtaining homogeneous structures which are impervious to gases and the acid electrolyte.

In acid fuel cells, graphite structures have been used to form part of the gas chambers for containing the fuel and oxidant in close spaced relationship with the cell electrodes. Acid cells and graphite plates are more completely described in a copending application of Dews et al having U.S. Ser. No. 789,777 and a filing date of Jan. 8, 1969 and assigned to the assignee of the present invention. Gas leakage impairs cell efficiency. It has been found that these structures should have a density of 1.8 g/cc or better to insure thatthe structure is impervious to hydrogen or other gases. These structures support the electrodes and direct the gas flow and are often used to form part of the electrical circuitry in a plurality of single cells. Heat management can be controlled by conducting heat through these structures to external fins or to a coolant. A unique combination of properties is required of an acid cell component.

To produce an economical and practical fuel cell system, emphasis has been placed on improving known cell components to provide similar improvements in cost, performance, and life of the fuel cell. Thus, improved structures and fabricating techniques are essential in the production of a better product in a competitive market. In addition, graphite structures which are characterized by high strengths, high conductiveities, and high density should have many useful applications.

SUMMARY OF THE INVENTION It is an object of this invention to provide a dense, conductive graphite structure and to provide a method of fabricating such a structure.

Another object of this invention is the production of a high density, homogeneous graphite body with improved structural, thermal, and electrical properties for use in acid fuel cells.

A further object of this invention is the method of producing coolant or separator plates for acid electrolyte fuel cells. Another aspect of this invention is the ability to mold thin graphite plates having complicated shapes in a simple operation.

It has now been found that the foregoing and related objects and advantages may be readily attained through the use of the method disclosed. In accordance with my invention, a method of forming a high density graphite structure comprises preparing a mixture of, by weight, five to twenty-five percent thermosetting phenolic resin binder and 75 to 95 percent sized powdered graphite, by mixing the resin binder with a liquid dispersant, blending the powdered graphite with the resin binder and dispersant to form a slurry, drying the slurry to drive off the dispersant, distributing a predetermined quantity of the graphite and binder mixture in a mold, applying less than about 100 psi pressure to the mold while heating the mold and contents to about 200 F, applying a full pressure of about 3,000 psi when the temperature reaches the selected value of approximately 200 F, heating the mold and contents to a temperature of 300-400 F, and cooling the mold and contents while full pressure is maintained. While commercially available powders produce an improved product, it has been observed that a structure with superior properties is obtained through the use of graphite powders selected so that the maximum particle size does not exceed 300 microns, and the particle distribution is selected in accordance with the limits specified in Table I hereinafter described.

Other objects and advantages of the present invention will become more apparent from the following detailed description of a preferred embodiment thereof and from the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a block diagram of a method of forming a high density graphite structure embodying my invention.

FIG. 2 is a graphical presentation of the graphite powder particle size distribution limits.

DESCRIPTION OF THE PREFERRED EMBODIMENT A method of forming a high density conductive v graphite structure is shown in FIG. 1. The invention contemplates the use of powder metallurgy techniques and comprises a novel process utilizing an improved graphite particle distribution. Using this process, I have discovered that graphite structures of superior strength, density, and conductivity can be molded in very intricate patterns and channels suitable for use in acid electrolyte fuel cells as support structures and coolant plates.

five to twenty-five weight percent of resin binder. It has also been discovered that a distribution of particle sizes as shown in Table l is important in producing graphite structures with densities as high as 2.0 g/cc.

TABLE i.-Pixii'rioi.n wi zion'r DISTRIBUTioN Broad Preferred The shaded band on FIG. 2 forms the basis for selecting the particle sizes, distribution and ranges. The particle weight distribution percentages tabulated represent the differences in cumulative weight percents which occur for the selected particle size range. For example, the weight percent at point B less the weight percent at A is 74 percent and represents the largest weight variation which might occur in the 60-100 micron range. The lowest variation of 20 percent is the difference in weight percent between points C and D. The preferred range for 60-100 microns is determined by the weight differences along the boundary of the band; i.e., the weight percent at point D less point A, which is 35, and the weight percent at point B less point C, which is 60. The optimum distribution is midway in the preferred range. No particle size should be greater than 300 microns and about 3-12 weight percent fine particles are necessary in achieving the desired structure.

After the slurry has been dried to drive off the dispersant, the powder may be screened to break up any agglomerates. Prior to loading the mold cavity with the graphite-resin powder composition, the mold surfaces are coated with a release agent to facilitate removal of the pressed structure. The mold is then loaded with a predetermined amount of powder to obtain the desired density and desired final thickness. Proper loading of the mold is important to insure uniformly dense and flawfree structures.

After the powder composition is loaded in the mold, a light pressure of less than about 100 psi is applied to the mold plungers while the mold and contents are heated to about 200 F. Preheating of the mold and contents prior to loading into a press is within the intended scope of the process. A nominal hold at 200 F, less than about 5 minutes for example, is satisfactory. The 200 F temperature appears to be the maximum temperature allowable before the resin curing starts to occur. Application of the low-pressure facilitates the escape of volatiles from the mold and contents. When the temperature reaches about 200 F, a full pressure of about 3,000 psi is applied. Heating is continued to between 300-400 F and thereafter the mold and contents are cooled while the full pressure ismaintained. After the structures are removed from the mold, they may be post-cured above the use temperature at around 325-400 F for approximately 6 hours to remove residual stresses.

Although the preferred process steps have been enumerated, certain temperature and pressure modifications may be made without sacrificing the product characteristics. For example, the initial pressing step may be carried out at pressures as low as a few psi while the temperature range associated with this step may vary from 180 to 220 F. The second step in the process may be carried out at temperatures between 300 and 400 F and at full pressures between 2,500 to 3,500 psi although about 3,000 psi is preferred. Hold periods depend upon the rate of heating and the thickness of the structure and need only be sufficient to permit the entire mass to uniformly attain the proper temperature.

In order to demonstrate the advantage of the present invention, a number of structures were fabricated in accordance with the described method and were compared to structures fabricated by the normal techniques.

Example I An percent graphite powder and 20 percent resin binder structure was constructed by blending the graphite powder into a mixture of a Resinox-Phenolic resin binder RS 1763 from the Monsanto Company and an isopropyl alcohol dispersant. The graphite particles were selected in accordance with the distribution as shown in FIG. 2. The slurry was dried to drive off the dispersant and loaded uniformly into a mold for pressing a complex graphite structure. A pressure of l00 psi was applied and the mold and contents were heated to about 200 F. Thereafter, 3,000 psi was applied and the temperature was raised to 385 F. The mold and contents were cooled while full pressure was maintained. The structure was post-cured and resulted in a plate with characteristics tabulated in column 1 of Table ll. Example II This same process was applied to a mixture of 80 percent graphite powder and 20 percent binder. The difference was that the powder was not selected in accordance with the schedule on FIG. 2. The mixture was the commercially available graphite powder and proprietary binder mixture designated as MG-2 available from the Carborundum Company. After post-curing, this plate has characteristics shown in column 2 of Table ll.

Example lll A graphite structure using the commercially available MG-Z graphite and binder mixture and the conventional pressing process demonstrate properties as shown in column 3 of Table II. The process consists essentially of pressing at a few hundred psi and heating to 300 F followed by a pressing operation at around 3,000 psi with molding temperatures around 400 F.

'riiiiia'n' Example I II III Density (glee) 2. 0 1. 1. 89 Compressive strength (lb/in. l517,000 17,000 12,000 Electrical resistivity (ohms-cm.) 4.3 10- 11 1Cr- Thermal conductivity (b.t.u./hr./lt./ F.):

Direction .L to pressing 32. 5 18 Direction ll to pressing 17 Although the invention has been shown and described with respect to a preferred embodiment, it should be understood by those skilled in the art that various changes and omissions in the form and detail may be made therein without departing from the spirit and the scope of the invention.

What we claim as novel and desire to secure by Letters Patent of the United States is:

l. A dense graphite structure, comprising:

five to 25 percent by weight thermosetting phenolic resin binder; and

75 to 95 percent by weight graphite powder, said graphite powder having a maximum particle size which does not exceed 300 microns, and having a particle size distribution in accordance with the following limits:

37 to 76 percent of the particles are in the 80 to 160 micron range,

20 to 74 percent of the particles are in the 60 to 100 micron range,

to 48 percent of the particles are in the 50 to 80 micron range,

three to 23 percent of the particles are in the 40 to 60 micron range, and

one to 12 percent of the particles are less than 50 microns.

2. A dense graphite structure as defined in claim 1, wherein the graphite particles have a particle size distribution in accordance with the following limits:

46 to 66 percent of the particles are in the 80 to I60 micron range,

35 to 60 percent of the particles are in the 60 to 100 micron range,

20 to 40 percent of the particles are in the 50 to micron range,

eight to l8 percent of the particles are in the 40 to 60 micron range, and

three to 12 percent of the particles are less than 50 microns.

3. A dense graphite structure as defined in claim 1, wherein the graphite particles h;ve a particle size distribution substantially in accordance with the following limits:

56 percent of the particles are in the 80 to micron range,

47 percent of the particles are in the 60 to 100 micron range,

30 percent of the particles are in the 50 to 80 micron range,

13 percent of the particles are in the 40 to 60 micron range, and

seven percent of the particles are less than 50 microns.

4. A dense graphite structure as defined in claim 1, wherein the structure has substantially the following physical characteristics:

a density of about 2.0 gm/cc, a compressive strength of about 15,000 lb./in. an electrical resistivity of about 4.3 X l0- ohm/cm., a thermal conductivity of about 32.5 BTU/hr./ ft/F when measured in a direction perpendicular to pressing and a thermal conductivity of about 17 BTU/hn/ft/F when mea sured in a direction parallel to pressing.

1 t! i i i 

2. A dense graphite structure as defined in claim 1, wherein the graphite particles have a particle size distribution in accordance with the following limits: 46 to 66 percent of the particles are in the 80 to 160 micron range, 35 to 60 percent of the particles are in the 60 to 100 micron range, 20 to 40 percent of the particles are in the 50 to 80 micron range, eight to 18 percent of the particles are in the 40 to 60 micron range, and three to 12 percent of the particles are less than 50 microns.
 3. A dense graphite structure as defined in claim 1, wherein the graphite particles h;ve a particle size distribution substantially in accordance with the following limits: 56 perCent of the particles are in the 80 to 100 micron range, 47 percent of the particles are in the 60 to 100 micron range, 30 percent of the particles are in the 50 to 80 micron range, 13 percent of the particles are in the 40 to 60 micron range, and seven percent of the particles are less than 50 microns.
 4. A dense graphite structure as defined in claim 1, wherein the structure has substantially the following physical characteristics: a density of about 2.0 gm/cc, a compressive strength of about 15,000 lb./in.2, an electrical resistivity of about 4.3 X 103 ohm/cm., a thermal conductivity of about 32.5 BTU/hr./ ft/*F when measured in a direction perpendicular to pressing and a thermal conductivity of about 17 BTU/hr./ft/*F when measured in a direction parallel to pressing. 